1. Field of the Invention
The invention relates to a method and a device for measuring speed by the Doppler effect, in particular for measuring a range of speeds in a flow of fluid, but equally for measuring speeds of moving solid bodies such as projectiles or ballistic missiles.
2. Description of the Prior Art
The components of the speed of an object illuminated by a laser sheet can be determined from, on the one hand, the intensity of the light diffused by that object and received directly by appropriate photoreceptors and, on the other hand, the intensity of the same light received by other appropriate photoreceptors via spectral filter means tuned to the frequency of the illuminating laser sheet. This is already known in the art (see in particular the documents EP-0506657 and AIAA 97-0498, 35th Aerospace Sciences Meeting and Exhibit, Reno, Nev., Jan. 6-10, 1997, xe2x80x9cPlanar Doppler Velocimetry performance in low-speed flowsxe2x80x9d by R. L. McKenzie). The illuminating laser sheet is typically emitted by an argon or Nd-YAG pulsed laser associated with a frequency doubler, the spectral filter means include a cell containing iodine vapor, which has an absorption line in the vicinity of the illuminating laser frequency and whose transmission is approximately 50% at that frequency, and the photoreceptors are of the CCD matrix type.
The function of the spectral filter means is to convert variations in the frequency of the light diffused by the object caused by the Doppler effect into variations in the intensity of the light picked up by the photoreceptors. Thus the intensity of the light picked up via the spectral filter means varies as a function of the speed of movement of the object. The ratio of the intensity picked up via the spectral filter means and the intensity picked up directly is calculated to obtain a normalized signal that varies as a function of the Doppler shift and a knowledge of which can be used to calculate the speed of the object in a direction in space.
Also, a portion of the laser beam is sampled and directed onto photoreceptors of the CCD matrix, on the one hand directly and on the other hand via the spectral filter means, to obtain a normalized signal (the previously cited ratio of the luminous intensities received directly and via the spectral filter means) that corresponds to a zero speed and is used for continuous compensation of variations in time of the laser frequency.
Accordingly, subject to prior calibration of the spectral transmission function of the filter means previously cited, the frequency difference due to the Doppler effect is determined from the normalized signal derived from the light diffused by a moving object and the zero speed normalized signal, and the speed of that object in the direction in space defined by the direction of the laser illumination and by the observation direction is then calculated. By effecting the observations in three different directions, three components of the speed vectors of objects moving in the observation field are obtained.
However, this prior art technique does not take into account the conversion drift of the spectral filter means, in particular as a function of temperature, in real time as a result of which the accuracy of the speed measurement proves insufficient.
One object of the invention is to solve this problem by simple, efficient and economic means.
To this end the invention proposes a method of measuring the speed of at least one object by the Doppler effect, including:
illuminating the object with a laser sheet derived from a laser beam,
measuring the intensity of the light diffused by the object, on the one hand directly and on the other hand via spectral filter means substantially tuned to the frequency of the laser illumination,
obtaining a normalized signal equal to the ratio of the intensity measured via the spectral filter means and the intensity directly measured,
measuring the intensity of a portion of the laser beam, on the one hand via the spectral filter means and on the other hand directly, and determining the ratio of the measured intensities to obtain a normalized signal corresponding to a zero speed, and
determining from said normalized signals a component of the speed of the object in a particular direction,
characterized in that it further includes:
measuring the intensity of at least one reference monochromatic luminous flux, on the one hand directly and on the other hand via the spectral filter means, said reference flux having a frequency differing from that of the laser beam by a known fixed amount, and
determining the ratio of the measured intensities of the reference flux to obtain a normalized reference signal corresponding to a fixed and known frequency difference and thus to a known speed different from zero.
This normalized reference signal, which corresponds to a known fixed frequency difference, is used to recalibrate in real time the transmission curve of the spectral filter means, compensating any drift thereof.
The frequency difference is advantageously chosen to cover a substantially linear portion of the transmission curve, starting from the point corresponding to a zero speed. It then becomes possible to determine accurately, by interpolation, the value of the Doppler shift that corresponds to a normalized signal derived from the light diffused by the object when that signal is between the normalized signal for the zero speed and the normalized signal for the reference flux.
This method advantageously includes measuring the intensities, on the one hand directly and on the other hand via the spectral filter means, of a plurality of reference monochromatic luminous fluxes whose frequencies differ from that of the laser beam by fixed and known amounts different from one to another and determining, for each reference flux, the ratio between said measured intensities to obtain normalized reference signals corresponding to different fixed and known frequency differences.
These frequency differences are advantageously staggered over a larger portion of the transmission curve of the spectral filter means, in particular over non-linear areas of that curve. For example, one effect of this is to double the frequency dynamic range, which results in a corresponding increase in the dynamic range of the measurable speeds.
In accordance with another feature of the above method, the or each reference flux is emitted in an intermediate image plane of optical means for forming an image of the object on a set of photodetectors. This avoids disturbing the field of speeds to be measured.
Advantageously, the or each reference flux is emitted by a substantially point source.
Thus only a very small portion of the image of this field is used to acquire the reference flux intensities.
The invention also proposes a device for measuring a speed of at least one object by the Doppler effect, including:
means for illuminating the object with a laser sheet derived from a laser beam,
optical means for forming an image of the object on two sets of photoreceptors whose output signals correspond to the received luminous intensity,
spectral filter means substantially tuned to the frequency of the laser beam and disposed between said optical means and one set of said two sets of photoreceptors,
means for directly and simultaneously directing a portion of the laser beam onto a first portion of each of said two sets of photoreceptors via the image forming optical means, to obtain normalized signals of the light diffused by the object and normalized signals corresponding to a zero speed, characterized in that it further includes:
means for generating at least one reference monochromatic luminous flux having a frequency different from that of the laser beam by a known fixed amount, and
means for directly and simultaneously directing said reference flux onto a second portion of each of said two sets of photoreceptors via the image forming optical means, to obtain a normalized reference signal corresponding to a fixed and known frequency difference.
The device according to the invention advantageously includes means for generating a plurality of reference monochromatic luminous fluxes whose frequencies differ from that of the laser beam by fixed and known amounts different from one to another and means for directing said reference fluxes onto separate portions of each of said two sets of photodetectors through the optical image forming means, to obtain normalized reference signals corresponding to fixed and known frequency differences.
In one preferred embodiment of the invention, the means for generating the reference flux or fluxes are acoustical-optical means such as a Bragg device.
These means generate reference fluxes whose frequencies are different from the frequency of the laser illumination by values equal at xc2x1nxcex94F, xcex94F being a fixed and known frequency difference, and n being an integer greater than zero.
The frequency differences are advantageously staggered regularly over the usable portion of the spectral transmission curve of the filter means previously cited.
According to other features of the invention:
the means for generating the references fluxes are connected by optical fibers to the image forming means,
the optical fibers have ends substantially in an intermediate image plane of the image forming means and oriented toward said photodetectors.
Generally speaking, the invention improves the accuracy of the measurement of the speed of an object by the Doppler effect and can be applied in fluid mechanics to measuring a range of speeds and in ballistics to measuring the speeds of moving objects, for example projectiles.